The present invention relates generally to gas turbine engines, and, more specifically, to a variable cycle engine for powering an aircraft at supersonic velocity in flight.
The common aircraft turbofan gas turbine engine includes a single stage fan driven by a low pressure turbine (LPT). A multistage axial compressor follows the fan for further pressurizing air which is mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the combustion gases in a high pressure turbine (HPT) that powers the compressor.
The fan and compressor are joined by independent drive shafts or spools to the corresponding rotors of the LPT and the HPT. In this way, the operating lines of the fan and compressor may be independently controlled during the various portions of the flight envelope including takeoff, climb, cruise, approach, and landing on the runway.
Turbofan engines are arranged in two distinct configurations. One configuration includes a short duct or nacelle surrounding the fan in a high bypass configuration having separate fan and core engine exhaust nozzles for separately discharging the air pressurized by the fan and the combustion gases generated in the core engine.
A second configuration of the turbofan engine includes a long duct or nacelle surrounding the fan and extending to the aft end of the engine in a common exhaust nozzle which discharges both the pressurized fan air and the combustion exhaust gases.
In both configurations, either a short or long bypass duct surrounds the core engine for bypassing or diverting a portion of the pressurized fan air around the core engine, including the high pressure compressor therein which has limited flow capability.
In the short nacelle configuration, the fan bypass duct is correspondingly short and terminates in an independent fan nozzle.
In the long duct configuration, the bypass duct extends from the fan to downstream of the LPT and typically rejoins the bypass air with the combustion exhaust flow prior to discharge in the common exhaust nozzle.
The common turbofan aircraft engine and its two independent rotors is typically configured for powering an aircraft at subsonic velocities well below Mach 1.
However, for supersonic military or commercial aircraft, the size, weight, and complexity of the turbofan engine increase substantially for producing the increased amount of propulsion thrust required for accelerating the aircraft to supersonic velocity greater than Mach 1, and maintaining that supersonic velocity during prolonged cruise operation. The supersonic business jet (SSBJ) is being designed for sustained supersonic cruise operation, yet requires commercially viable efficiency of the engine, and regulatory acceptable levels of exhaust noise.
Noise generation in a supersonic aircraft is a significant design problem for meeting various governmental noise regulations, typically most severe in the immediate vicinity of an airport.
Accordingly, the prior art is replete with various configurations of variable cycle turbofan engines specifically configured for powering aircraft at supersonic velocity. The size, weight, and complexity of these various variable cycle turbofan engines vary dramatically, along with the aerodynamic efficiency thereof and the level of noise generated during operation. Substantial compromises in the design of the various components of the supersonic aircraft engine must be made in an attempt to balance the competing design objectives for obtaining high performance.
One form of variable cycle engine includes a FLADE, which is an acronym for “fan on blade.” The FLADE is a special form of fan that includes relatively large fan blades having a radially outer tip extension defined by a part-span integral shroud. The FLADE airfoil, or outer portion of the fan blade above the shroud is specifically configured in aerodynamic profile for efficiently pressurizing tip air which flows downstream through a corresponding annular bypass duct surrounding the core engine. This FLADE bypass air may then be used in various forms of specialized exhaust nozzles for reducing acoustic noise during desired portions of the flight envelope.
A substantial problem in incorporating FLADEs in turbofan engines is the additional centrifugal force generated thereby during operation which must be accommodated by the inner airfoil and supporting rotor disk. The outer FLADE airfoil and its integral inner shroud create large centrifugal loads during rotary operation of the fan, and therefore require a thicker inner airfoil and larger supporting rotor disk for carrying the centrifugal loads within suitable stress limits for ensuring long life of the fan.
The thicker fan airfoil in turn decreases aerodynamic efficiency and performance of the airfoil, which correspondingly reduces overall efficiency of the engine.
The FLADE may therefore be used to provide pressurized air for acoustic nozzles, which allows for a higher fan pressure ratio in the turbofan engine at noise levels equivalent to larger, lower fan pressure ratio engine cycles. In subsonic cruise configurations, a FLADED mixed flow turbofan engine can show a performance improvement relative to a FLADED variable cycle engine, but only marginally better performance relative to the conventional mixed flow turbofan engine.
The FLADED engine may enjoy the benefit of increased thrust per unit airflow at the considerable expense of the increase in centrifugal loads from the FLADE airfoils, and corresponding increase in weight of the engine for the accommodation thereof, as well as aerodynamic performance penalties due to the thicker supporting fan airfoil below the FLADE.
Furthermore, the introduction of the FLADE in a turbofan engine typically includes inlet guide vanes (IGVs) before the FLADED fan stage, as well as outlet guide vanes (OGVs) following the FLADED stage. These guide vanes are used to increase aerodynamic efficiency, but require a corresponding increase in length of the engine, and corresponding increase in weight and complexity.
The dilemma then facing the engine designer in configuring a practical supersonic aircraft engine is the delicate balance between aerodynamic configuration, mechanical strength, exhaust noise, size, weight, and complexity of the various components of the turbofan engine which are typically mutually interrelated.
Accordingly, it is desired to provide a supersonic aircraft turbofan engine having improved performance and efficiency and noise attenuation.